Heat dissipation apparatus and method for attaching a heat dissipation apparatus to an electronic device

ABSTRACT

An apparatus for dissipating heat generated by an electronic device and a method for mounting the apparatus to an electronic device are disclosed. The apparatus includes a heat transfer body which is mounted in proximity to an electronic device. In addition, the apparatus includes an adhesive distributed on the heat transfer body, which affixes the heat transfer body in proximity to the electronic device. The adhesive is distributed on the heat transfer body such that heat transfer from the electronic device to the heat transfer body occurs substantially independently from the adhesive. The apparatus further includes a thermally conductive material disposed between the electronic device and the heat transfer body. The thermally conductive material is selected to maximize heat transfer from the electronic device to the heat transfer body.

BACKGROUND OF THE INVENTION

1. Technical Field:

The present invention relates in general to thermal management ofelectronic devices and in particular to a heat dissipation apparatus foran electronic device. Still more particularly, the present inventionrelates to a heat sink apparatus and method for attaching the heat sinkapparatus to an electronic device, wherein the heat sink apparatus ismechanically attached to the electronic device by adhesive and thermallycoupled to the electronic device by a thermally conductive material.

2. Description of the Related Art:

As the performance requirements for computers and other electronicequipment increase, the integrated circuit (IC) components comprisingthe electronic equipment operate at higher power and are manufactured atincreased device densities. As a result, greater emphasis is beingplaced on the utilization of heat sinks and other means for managing thethermal environment of the IC components.

A conventional package for an IC chip includes a substrate to which thechip is electrically connected and a cap which seals the chip within thepackage. In addition, the package typically includes a thermal pastesandwiched between the cap and the upper chip surface which conductsheat from the chip to the cap. Packages for high-power chips oftenutilize a heat sink attached to the package cap to enhance theefficiency of heat transfer from the package cap to the surroundingenvironment, thereby maintaining the temperature of the chip within therecommended operating temperature range.

As will be appreciated by those skilled in the art, heat sinks can beattached to IC packages by a variety of means, including clamps, screws,and other hardware, as well as thermally conductive adhesives. Becauseof the package-dependence of clamp or screw-mounted heat sinks and theadditional labor required to attach clamp and screw-mounted heat sinksto packages, electronic device manufacturers often prefer utilizingadhesive-mounted heat sinks in order to minimize production costs.However, because materials having desirable adhesive propertiestypically do not have a high thermal conductivity, it is difficult toprovide a high performance heat sink at a low cost.

Consequently, it would be desirable to provide an improved heat sinkapparatus and method for attaching a heat sink apparatus to anelectronic device which enable a high thermal performance heat sinkapparatus to be assembled with minimal labor utilizing low-costmaterials.

SUMMARY OF THE INVENTION

It is therefore one object of the present invention to provide animproved method and apparatus for thermal management of an electronicdevice.

It is another object of the present invention to provide an improvedheat dissipation apparatus for an electronic device.

It is yet another object of the present invention to provide an improvedheat sink apparatus and method for attaching the heat sink apparatus toan electronic device, wherein the heat sink apparatus and electronicdevice are mechanically attached by adhesive and thermally coupled by athermally conductive material.

The foregoing objects are achieved as is now described. An apparatus fordissipating heat generated by an electronic device and a method formounting the apparatus to an electronic device are disclosed. Theapparatus includes a heat transfer body which is mounted in proximity toan electronic device. In addition, the apparatus includes an adhesivedistributed on the heat transfer body, which affixes the heat transferbody in proximity to the electronic device. The adhesive is distributedon the heat transfer body such that heat transfer from the electronicdevice to the heat transfer body occurs substantially independently fromthe adhesive. The apparatus further includes a thermally conductivematerial disposed between the electronic device and the heat transferbody. The thermally conductive material is selected to maximize heattransfer from the electronic device to the heat transfer body.

The above as well as additional objects, features, and advantages of thepresent invention will become apparent in the following detailed writtendescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself however, as well as apreferred mode of use, further objects and advantages thereof, will bestbe understood by reference to the following detailed description of anillustrative embodiment when read in conjunction with the accompanyingdrawings, wherein:

FIG. 1A illustrates a capped single chip module to which a preferredembodiment of the heat sink apparatus of the present invention isattached;

FIG. 1B illustrates a closeup view of the adhesive utilized to attachthe heat sink apparatus to the capped single chip module in FIG. 1A;

FIG. 2 depicts a bottom plan view of a first preferred embodiment of theheat sink illustrated in FIG. 1A;

FIG. 3 illustrates a bottom plan view of a second preferred embodimentof the heat sink illustrated in FIG. 1A;

FIG. 4 depicts a bottom plan view of a heat sink having a recess aroundthe perimeter of the bottom surface according to the present invention;

FIG. 5A and 5B illustrate bottom plan views of two heat sink apparatuseshaving one or more isolation channels formed therein according to thepresent invention;

FIG. 6 depicts a capless single chip module to which a preferredembodiment of the heat sink apparatus of the present invention isattached;

FIG. 7A and 7B illustrate elevation and top plan views of asurface-mounted IC chip to which a heat sink is attached according tothe present invention;

FIG. 8 is a flowchart depicting a preferred embodiment of the method ofattaching a heat sink apparatus to an electronic device according to thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

With reference now to the figures and in particular with reference toFIG. 1A, there is illustrated a single chip module to which a heat sinkis attached according to the method and system of the present invention.As illustrated, single chip module (SCM) 10 comprises a singleintegrated circuit chip 12 which is electrically connected to substrate14 by chip connection 16. In the depicted embodiment, chip connection 16comprises microballs of non-eutectic solder which form a controlledcollapsible chipconnection (C4). Electrical signals transmitted tosubstrate 14 by chip connection 16 are conducted to other electronicdevices mounted on circuitcard 18 by multiple pins 20. Chip 12 is sealedwithin SCM 10 by cap 22, which is formed of aluminum, thermoplastic,ceramic, or other suitable material. As is typical in IC packaging, heatgenerated by chip 12 is transferred to cap 22 through a layer of thermalpaste 24, which intimately contacts the upper surface of chip 12 and thelower surface of cap 22. The design of SCM 10 forms no part of thepresent invention and isprovided for illustrative purposes only. Fromthe following description, those skilled in the art will appreciate thatthe present invention is package-independent and is therefore equallyapplicable to ball grid array(BGA), ceramic column grid array (CCGA),quad flat package (QFP), pin grid array (PGA), capless SCMs, multi-chipmodules (MCMs) or any other electronic device package to which a heatsink may be attached.

To maintain chip 12 within its recommended operating temperature range,SCM10 is provided with heat sink 26, which in the depicted embodimentcomprises a finned heat sink. Although heat sink 26 is illustrated as afinned heat sink, other heat sink configurations such as pinned,impingement, staggered pins, and offset-strip fins may be utilized.According to the present invention, heat sink 26 is mechanicallyattached to SCM 10 by adhesive 28, which in the depicted embodiment isdisposed between heat sink 26 and SCM 10 at a number of locations.Because adhesive28 is not utilized as the primary thermal interfacebetween SCM 10 and heatsink 26, adhesive 28 may be selected based uponits adhesive properties andnot its thermal conductivity. Thus,pressure-sensitive adhesive, UV-sensitive adhesive, epoxy, or any othersuitable type of adhesive may be utilized. Of course, the adhesiveselected for use in a particular application should have a modulus andstrength appropriate for that application.

As depicted in FIG. 1A, adhesive 28 is preferably disposed adjacent tothe edges of SCM 10 and away from the primary heat transfer region ofcap 22, which is typically the center. Referring now to FIG. 1B, thereis depicteda closeup view of a corner of heat sink 26, which illustratesadhesive 28 housed within a recess 40. Although recess 40 is preferablyconfigured as a notch in the perimeter of the bottom surface of heatsink 26, those skilled in the art will appreciate that a number ofrecess geometries, including bevels (chamfers) or wells, may beemployed. Regardless of whichrecess geometry is utilized in a particularapplication, recess 40 must have sufficient depth to accommodate theminimum amount of adhesive required to achieve the shear bondingstrength required in that application. For example, for a typicaladhesive the minimum thickness which provides acceptable shear strengthcharacteristics is approximately 0.010 in.

With reference now to FIGS. 2 and 3, there are depicted bottom planviews of two alternative embodiments of heat sink 26. FIG. 2 illustratesan embodiment of heat sink 26 having a recess 40a at each of the fourcornersof heat sink 26. Thus, according to a first preferred embodimentof heat sink 26, heat sink 26 is mechanically attached to SCM 10 byadhesive 28 disposed within each of recesses 40a. As illustrated in FIG.3, in a second preferred embodiment of heat sink 26, recesses 40b extendthe length of each of two opposing sides of heat sink 26. Thus, in thesecond preferred embodiment, heat sink 26 is affixed to SCM 10 by atleast one bead of adhesive disposed within each of recesses 40b. Thedesign of the second preferred embodiment of heat sink 26 iseconomically advantageous because heat sink 26, including recesses 40b,can be formed by extrusion.

Referring now to FIG. 4, there is depicted a bottom plan view of heatsink 50 which has a recess 52 around the perimeter of its lower surface.Because of the large surface area within recess 52 to which adhesive canbe applied, heat sink 50 is advantageously utilized in applicationswhich require a large or heavy (e.g., copper) heat sink, which aresubjected to vibration, or which require a particularly strong adhesivebond.

Referring again to FIG. 1A, heat is transferred between cap 22 and heatsink 26 through thermally conductive material 30. Thermally conductivematerial 30 is preferably a low viscosity oil (e.g., poly(α-olefin)),graphite, or other material selected for its high thermal conductivity.The interface gap between heat sink 26 and cap 22, which is determinedin part by the volume of thermally conductive material30, the amount ofadhesive 28 utilized, and the flatness and finish of heatsink 26 and cap22, is preferably only a few microns or less wide to minimize thermalresistance. Because the interface gap between heat sink 26 and cap 22 issubstantially narrower than the gaps in prior art heat sink apparatuseswhich utilize a thermally conductive adhesive to transferheat between anelectronic device and a heat sink, the thermal performance of thepresent invention is greatly enhanced. As described below with respectto FIGS. 5A and 5B, heat sink 26 also optionally includes eitherisolation channels 54 or isolation channel 32 to minimize contactbetween thermally conductive material 30 and adhesive 28.

With reference now to FIGS. 5A and 5B, there are illustrated bottom planviews of third and fourth alternative embodiments of heat sink 26 ofFIG. 1A which depict one or more isolation channels formed within thebottom surface of heat sink 26. Referring first to FIG. 5A, adhesive 28within each of recesses 40c is isolated from thermally conductivematerial 30 by an isolation channel 54 formed within the bottom surfaceof heat sink ;26.Isolation channels 54 restrict contact betweenthermally conductive material 30 and adhesive 28 because the open volumeof isolation channels 54 is less resistive to fluid flow than the narrowinterface gap between the lower surface of heat sink 26 and cap 22.Thus, portions of thermally conductive material 30 flowing intoisolation channels 54 will tend to fill isolation channels 54 ratherthan flow toward recesses 40c. Isolationof thermally conductive material30 from adhesive 28 may or may not be required depending upon thechemical properties of the selected thermally conductive material 30 andadhesive 28. Accordingly, isolation channels 54are depicted in FIG. 1Autilizing dashed-line illustration. However, in many applicationsisolation of thermally conductive material 30 and adhesive 28 isrequired at least prior to curing adhesive 28 in order to preventdegradation of adhesive 28. As will be appreciated by those skilled inthe art, the depth and volume of isolation channels 54 are dependent ona variety of factors, including the volume and viscosity of thermallyconductive material 30. For example, in a typical application isolationchannels 54 are 0.020 in deep and 0.020 in wide. Isolation channels 54can be machined into the bottom surface of heat sink 26 ormayalternatively be formed during casting of heat sink 26.

With reference now to FIG. 5B, the fourth alternative embodiment of heatsink 26 is provided with a isolation ring 56 within the bottom surfaceof heat sink 26. As will be appreciated by those skilled in the art,thermally conductive material 30 disposed between cap 22 and heat sink26 expands as the temperature of SCM 10 increases. In order to preventthermally conductive material 30 from contacting adhesive 28 withinrecesses 40a or from escaping from the interface gap between cap 22 andheat sink 26, isolation ring 56 is designed with sufficient volume toaccommodate the expanded volume of thermally conductive material 30.Thus,isolation ring 56 serves as a thermal expansion reservoir forthermally conductive material 30 in applications where this is required.As illustrated within FIGS. 1A and 5A, isolation ring 56 communicateswith the environment of SCM 10 via inlets 32. In some embodiments of thepresent invention, heat sink 26 is preferably affixed to cap 22 prior tointroducing thermally conductive material 30 in order to maximize thebonding strength of adhesive 28. In these embodiments of the presentinvention, thermally conductive material 30 is introduced into inlets 32after the attachment of heat sink 26 and is drawn into isolation ring 56(and subsequently the interface gap) by capillary action. Like isolationchannels 54 depicted in FIG. 5A, isolation ring 56 and inlets 32 may beformed during the casting of heat sink 26 or alternatively by machining.Again, the depth and geometry of inlets 32 and isolation ring 56representa design choice influenced by other features of the heat sinkapparatus, such as the selected thermally conductive material 30 and theinterface gap spacing.

Referring now to FIG. 6, there is illustrated a capless single chipmodule to which a heat sink is attached according to the presentinvention. As indicated by like reference numerals, several componentswithin capless SCM 60 are similar to those within SCM 10. However, sinceSCM 60 is capless, heat sink 62 is attached directly to the upper(inactive) surfaceof chip 12 by adhesive 64. As is other embodiments ofthe present invention, heat is transferred from chip 12 to heat sink 62through a layer of thermally conductive material 66. As is well-known tothose skilled in the art, the upper surface of chip 12 will be distorteddue to chip fabrication and attachment processes as well as at operatingtemperatures. To maximize the efficiency of thermal transmission fromchip12 to heat sink 62, the surface of heat sink 62 mated to chip 12 canbe contoured to approximate the upper surface of chip 12 as distorted atoperating temperatures. Thus, by machining or molding the mating surfaceof heat sink 62, a relatively uniform gap can be maintained between heatsink 62 and chip 12.

With reference now to FIGS. 7A and 7B, there are illustrated elevationand top plan views of a preferred embodiment of the present invention inwhicha nondirectional heat sink is attached to a surface-mounted ICchip. As illustrated, chip 70 is electrically and mechanically connectedto substrate or circuit card 72 by chip connection 74. Chip 70 isunderfilledwith an adhesive to protect chip connection 74 and tostrengthen the mechanical connection of chip 70 and circuit card 72.Like other embodiments of the present invention, chip 70 is equippedwith a heat sink76 to dissipate heat generated by chip 70. Heat isconducted from the uppersurface of chip 70 to heat sink 76 throughthermally conductive material 77. As depicted in FIGS. 7A and 7B, thediameter of the lower surface of heat sink 76 is preferably larger thanat least one dimension of the top surface of chip 70 such that arcuateregions 78 of the lower surface of heat sink 76 extend beyond chip 70.Heat sink 76 is attached to circuit card 72 and held in proximity tochip 70 by adhesive 79, which preferably comprises the same adhesiveutilized to underfill chip 70.

Referring now to FIG. 8, there is depicted a flowchart of a preferredembodiment of the method for attaching a heat sink to an electronicdeviceaccording to the present invention. To maximize processefficiency, the method of the present invention is preferably performedby an automated assembly line; however, the method depicted in FIG. 8can also be performed manually. As illustrated, the process begins atblock 80 and thereafter proceeds to block 82, which depicts receiving anelectronic device module and a heat sink at a dispensing station. Topromote a strongadhesive bond, the electronic device module and the heatsink are preferably clean when received at the dispensing station.Furthermore, theelectronic device module is preferably mounted on acircuit card and testedprior to receipt at the dispensing station.Testing the electronic device module in situ prior to attaching the heatsink decreases the number of heat sinks which must subsequently beremoved to replace a defective electronic device module.

Next, the process proceeds to block 84, which illustrates the dispensingstation dispensing adhesive at a number of locations. The dispensingstation preferably utilizes positive displacement dispensing in order toprovide a controlled amount of adhesive at each of the locations atwhich adhesive is dispensed. In embodiments of the present invention inwhich the heat sink is attached directly to the electronic devicemodule, adhesive is dispensed on either the mating surface of theelectronic device module or the mating surface of the heat sink. Inembodiments of the present invention such as that illustrated in FIGS.7A and 7B, the adhesive is dispensed both beneath the edges of the chipto encase the chip connection and at locations surrounding the perimeterof the chip forsubsequent attachment of the heat sink. The process thenproceeds to block 86, which depicts adding an activator to each portionof dispensed adhesive in order to trigger a bonding reaction. Asdepicted by dashed-line illustration, block 86 is performed only if theselected adhesive utilizes an activator.

The process proceeds from either block 84 or block 86 to block 88, whichillustrates the dispensing station dispensing a thermally conductivematerial on the surface of at least one of the electronic device moduleand the heat sink. Again, positive displacement dispensing is preferablyutilized to control the amount of thermally conductive materialdispensed,and consequently to control the interface gap between theelectronic devicemodule and the heat sink. Although positivedisplacement dispensing is preferably employed, those skilled in the artwill appreciate that in alternative embodiments of the present inventionin which the thermally conductive material is not a fluid, alternativeprocess steps are performed. For example, in embodiments of the presentinvention in which the thermally conductive material is configured assheet stock, block 88 illustrates laying a sheet of the thermallyconductive material on the mating surface of either the heat sink or theelectronic device module. Furthermore, block 88 is performed atdifferent times in the process sequence depending upon the adhesive,thermally conductive material, and heat sink configuration employed. Forexample, in some embodiments of the present invention, block 88 may beperformed prior to block 84. Alternatively, as described above withreference to FIG. 5B, the thermallyconductive material can be injectedinto the interface between the heat sink and electronic device modulesubsequent to mating the heat sink and electronic device module inembodiments of the present invention which include inlets 32 (i.e.,block 88 can be performed subsequent to block 92).

The process proceeds from block 88 to block 90, which depicts mating theheat sink and electronic device module. Next, the process proceeds toblock 92 which depicts curing the adhesive. As will be appreciated bythose skilled in the art, the type of curing required depends upon thetype of adhesive selected. For example, if a pressure-sensitive adhesiveis utilized, the curing step illustrated at block 92 entails maintainingapressure of 10 psi for two minutes. Alternatively, if the adhesive isan epoxy, the adhesive is cured by heating the electronic device moduleand heat sink apparatus within an oven for two hours at 150° C. and apressure of 10 psi. As another example, if the adhesive is aUV-sensitive adhesive, the adhesive is cured by irradiating the adhesivewith ultraviolet radiation. The present invention enables UV-sensitiveadhesiveto be utilized since the adhesive is accessible to a UV sourcearound the perimeter of the electronic device module. Thereafter, theprocess terminates at block 94.

As has been described, the present invention provides an improvedapparatusfor dissipating heat generated by an electronic device and animproved method for attaching a heat sink apparatus to an electronicdevice. By utilizing both a high performance thermally conductivematerial to transfer heat from the electronic device to the heat sinkand an inexpensive and durable adhesive attachment, the presentinvention provides improved thermal integrity at a reasonable cost.

While the invention has been particularly shown and described withreference to a preferred embodiment, it will be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention.

What is claimed is:
 1. An apparatus comprising:a heat transfer bodymounted in proximity to an electronic device, said heat transfer bodyhaving a surface; an adhesive distributed on said heat transfer body,wherein said adhesive affixes said surface of said heat transfer body inproximity to said electronic device, said adhesive being distributedsuch that heat transfer from said electronic device to said heattransfer body occurs substantially independently of said adhesive; and athermally conductive material disposed between and in contact with saidelectronic device and said surface of said heat transfer body, whereinsaid thermally conductive material is a liquid at an operatingtemperature of said electronic device; wherein said surface of said heattransfer body is in contact with both said thermally conductive materialand said adhesive, said surface having at least one channel formedtherein that is located between regions of said surface contacted bysaid adhesive and regions of said surface contacted by said thermallyconductive material such that said at least one channel limits contactbetween said thermally conductive material and said adhesive.
 2. Theapparatus of claim 1, wherein said thermally conductive materialcomprises a low viscosity oil.
 3. The apparatus of claim 1, wherein saidadhesive is distributed at a plurality of noncontiguous locations onsaid heat transfer body.
 4. The apparatus of claim 1, wherein saidsurface of said heat transfer body includes at least one recess formedtherein at one or more locations corresponding to one or more locationsat which said adhesive is distributed, said at least one recess having abonding surface to which said adhesive is affixed.
 5. The apparatus ofclaim 4, wherein said at least one recess comprises a chamfer formed ata perimeter of said surface of said heat transfer body.
 6. The apparatusof claim 5, said electronic device including a mounting surface uponwhich said heat transfer body is mounted, wherein said adhesive forms acontinuous bead adjacent to a perimeter of said mounting surface of saidelectronic device.
 7. The apparatus for dissipating heat of claim 1,wherein a volume of said at least one channel is greater than volumetricexpansion of said thermally conductive material within an operatingrange of temperatures of said electronic device.
 8. The apparatus ofclaim 1, and further comprising said electronic device, wherein saidelectronic device includes:an integrated circuit chip; a packageenclosing said integrated circuit chip, said package having a mountingsurface that supports said heat transfer body, wherein said mountingsurface is in direct contact with said thermally conductive material;and a connection for connecting said electronic device to a substrate.9. The apparatus of claim 8, said mounting surface having a first regionto which said adhesive is affixed and a second region that directlycontacts said thermally conductive material.
 10. The apparatus of claim9, wherein said heat transfer body is substantially coextensive withsaid mounting surface of said package.
 11. The apparatus of claim 1, andfurther comprising said electronic device, wherein said electronicdevice comprises an integrated circuit chip having a mounting surfacethat supports said heat transfer body, said mounting surface having afirst region to which said adhesive is affixed and a second region thatdirectly contacts said thermally conductive material.
 12. The apparatusof claim 11, wherein said heat transfer body is substantiallycoextensive with said mounting surface of said integrated circuit chip.